Ventilation using mechanical ventilators is widely accepted as an effective form of therapy and means for treating patients who require respiratory assistance. Ventilation is the process of delivering oxygen to and washing carbon dioxide from the alveoli in the lungs. A medical ventilator delivers gas to a patient's respiratory tract and is often required when the patient is unable to maintain adequate ventilation. Mechanical ventilation is the single most important therapeutic modality in the care of critically ill patients. Known ventilators typically include a pneumatic system with variable pressure, flow and volume characteristics that delivers and extracts air and/or gas to the patient and a control system (typically consisting of a microprocessor with a keypad and a display) that provides the interface to the treating clinician. Optimal support of the patient's breathing requires adjustment by the clinician of the pressure, flow, and volume of the delivered gas as the condition of the patient changes. Such adjustments, although highly desirable, are difficult to implement with known ventilators because the control system demands continuous attention and interaction from the clinician based on the patient's condition.
Medical ventilator systems have been used for many years to provide supplemental oxygen support to patients unable to breathe normally. These medical ventilators typically include a source of pressurized oxygen, a flow generator, an air filter, a mask, an air delivery conduit connecting the flow generator to the mask, various sensors and a microprocessor-based controller. The flow generator may include a servo-controlled motor and an impeller. The flow generator may also include a valve capable of discharging air to atmosphere as a means for altering the pressure delivered to the patient as an alternative to motor speed control. The sensors typically measure motor speed, gas volumetric flow rate and outlet pressure. The apparatus may optionally include a humidifier in the air delivery circuit. The controller may also include data storage capacity with or without integrated data retrieval and display functions.
Most modern ventilators allow the clinician to select and use several modes of inhalation either individually or in combination via the ventilator setting controls that are common to the ventilators. These modes can be defined in three broad categories: spontaneous, assisted or controlled. During spontaneous ventilation without other modes of ventilation, the patient breathes at his own pace, but other interventions may affect other parameters of ventilation including the tidal volume and the baseline pressure (above ambient) within the system. In assisted ventilation, the patient initiates the inhalation by lowering the baseline pressure by varying degrees, and then the ventilator “assists” the patient by completing the breath by the application of positive pressure. During controlled ventilation, the patient is unable to breathe spontaneously or initiate a breath, and is, therefore, dependent on the ventilator for every breath. During spontaneous or assisted ventilation, the patient is required to “work” (to varying degrees) by using the respiratory muscles in order to breath.
The simplest way to look at mechanical ventilation is as a way to keep the blood gases normal. The most relevant parameters of a normal blood gas are hydrogen ion concentration (pH), partial pressure of carbon dioxide (pCO2) and partial pressure of oxygen (pO2). There are several other values, but many of these are calculated and/or not reflective of pulmonary function which is what is being controlled with mechanical ventilation. Hydrogen ion concentration and partial pressure of carbon dioxide are closely related and are affected by minute ventilation (respiratory rate times tidal volume or RR×TV). Partial pressure of oxygen is governed by oxygen delivery and ventilation and perfusion (V and Q) match. Because CO2 rapidly diffuses across the alveolar space, the more air that is moved into and out of the lungs, the more rapidly the CO2 can be removed.
Oxygen delivery is dependent on ventilation and perfusion match and partially determined by a patient's fraction of inspired oxygen (FiO2) and is partly related to the patient's airway recruitment. Airway recruitment is indirectly reflected in the patient's mean airway pressure (MAP). By increasing the patient's mean airway pressure, airway recruitment can be increased (although this is not a linear relationship). Mean airway pressure is a function of the positive end expiratory pressure (PEEP) and a fraction of the peak inspiratory pressure (PIP or Pmax).
Oxygen therapy or the supplemental oxygen treatment of patients on mechanical ventilation is crucial in maintaining the patients' oxygen levels in the normal range. Oxygen therapy is defined as the administration of oxygen at concentrations greater than ambient air (approximately 21% oxygen). The percentage of oxygen in the air inhaled by a patient, either on or off the ventilator, is called the fraction of inspired oxygen (FiO2). FiO2 ranges are from 21% (e.g. ambient room air) to 100% (e.g. pure oxygen). Typically, an FiO2 not exceeding 0.25-0.35 is needed and a 0.05 variation in FiO2 is generally clinically acceptable. For most patients, a precise or high inspiratory oxygen fraction (FiO2) is not required. In some ventilators, FiO2 is increased by the attachment of an oxygen (O2) accumulator to the gas entry port. Alternatively, supplemental O2 may be titrated into the inspiratory limb of the ventilator circuit between the ventilator and the humidifier or, during noninvasive positive pressure ventilation (NPPV), via oxygen tubing connected directly to the mask.
Monitoring the arterial oxygen saturation (SpO2) using pulse oximetry is a simple non-invasive method (i.e., the skin does not have to be broken to perform the test ), which allows health care providers to monitor the oxygen-bound hemoglobin of a patient's blood. Pulse oximetry measures the ratio of oxygenated hemoglobin to the total amount of hemoglobin, i.e., the percentage of haemoglobin (Hb) which is saturated with oxygen, or the amount of oxygen in the blood of the arteries. The pulse oximeter uses two wavelengths of light (650 nm (red) and 805 nm (infrared)) originating from a probe and passing through the patient's skin (preferably the patient's finger, ear lobe or toe). The light is partly absorbed by haemoglobin, by amounts which differ depending on whether the haemoglobin is saturated or desaturated with oxygen. A sensor measures the amount of light the tissue absorbs and the output from the sensor is linked to a microprocessor. By calculating the light absorption at the two wavelengths, the microprocessor can compute the proportion of haemoglobin which is oxygenated.
Based upon the ratio of absorption of the red and infrared light caused by the difference in color between oxygen-bound (red) and unbound (blue) hemoglobin in the capillary bed, an approximation of oxygenation can be made. The pulse oximeter displays the percentage of Hb saturated with oxygen together with an audible signal for each pulse beat, a calculated heart rate and in some models, a graphical display of the blood flow past the probe (plethysmograph). The pulse oximeter provides a means to determine how well the patient is being oxygenated. The oximeter is dependant on a pulsatile flow and produces a graph of the quality of flow. Where flow is sluggish (e.g., hypovolaemia or vasoconstriction), the pulse oximeter may be unable to function. The computer within the oximeter is capable of distinguishing pulsatile flow from other more static signals (such as tissue or venous signals) to display only the arterial flow.
The previously known medical ventilators generally require a user interface to successfully adjust and maintain the patient's FiO2. This is most likely due to the complex and non-linear behavior of the human body's response to various physiological stimuli. Accordingly, there is a need for a ventilator system that can effectively automatically control the fraction of inspired oxygen based on the patient's SpO2.